Learn Hardcore Croatian: Na prvom satu nisam bio razumio razliku između rečenica „radio sam“ i „bio sam radio“. - In the first lesson I hadn’t understood the difference between the sentences “I worked” and “I had worked”.

nisam = ne (not) + sam (I am) → 1st person singular of biti (to be), used as an auxiliary
bio = past participle of biti (been)
razumio = past participle of razumjeti (understood)

Literally, it’s something like I was not been understood in structure, but the meaning is I had not understood.

So:

nisam razumio ≈ I didn’t understand
nisam bio razumio ≈ I hadn’t understood (before some later point)

Is nisam bio razumio common in modern Croatian, or is it old‑fashioned?

The pluperfect (nisam bio razumio, bio sam radio, etc.) is:

Grammatically correct
Still understood by everyone
But rare in everyday spoken Croatian

In most normal conversation, people would usually say:

Na prvom satu nisam razumio razliku…

and not bother with the pluperfect. The pluperfect appears more in:

Written language (literature, formal narration)
Stylized or very careful speech
Certain regional/dialectal uses

So your sentence is fine, but in casual speech it would often be simplified to nisam razumio.

What is the difference in meaning between radio sam and bio sam radio?

Both come from raditi (to work / to be doing something), but they express different time relations:

radio sam = simple past (perfect)
- Roughly: I worked / I was working / I have worked
- Just says that the action happened in the past.
bio sam radio = pluperfect
- Roughly: I had worked / I had been working (before something else)
- Emphasizes that the action was already completed before another past event.

In your sentence, the speaker is saying he didn’t understand the difference between:

a normal past action (radio sam)
and a past action prior to another past point (bio sam radio).

So in practice, when would I actually use bio sam radio instead of radio sam?

Use bio sam radio when you clearly want to show that something happened earlier than some other past event.

Example:

Kad je on došao, ja sam radio.
→ When he arrived, I was working. (both in the past, overlapping)
Kad je on došao, ja sam bio radio cijeli dan i već sam bio umoran.
→ When he arrived, I had worked all day and I was already tired.
(my working all day is clearly before the moment he arrived)

In everyday Croatian, many speakers would still express the second idea with just radio sam, relying on context:

Kad je on došao, radio sam cijeli dan i već sam bio umoran.

So bio sam radio is more explicit and more “textbook-correct” for the pluperfect, but often avoided in casual talk.

Why is the auxiliary sam after radio in radio sam, but in the middle in bio sam radio?

Croatian has a special rule for short forms like sam, si, je, smo, ste, su, nisam, nije…. They are clitics, and they like to stand in the second position of the clause (more precisely, second syntactic unit).

In radio sam, the clause is very short, so radio is first and sam comes second.
In bio sam radio, the clause is:
- bio (first element)
- sam (second element, as required by the clitic rule)
- radio (rest)

You cannot normally put sam at the very beginning:

✗ Sam radio (wrong in neutral Croatian)
✓ Radio sam

Likewise:

✗ Sam bio radio (wrong)
✓ Bio sam radio

In na prvom satu, why do we use na and not u?

Both na and u can translate to in/on/at, but their usage is partly fixed by convention.

na satu literally: on/at the lesson/class
- This is the idiomatic phrase in Croatian for being in a class/lesson.
Saying u satu would sound wrong; it would literally mean inside an hour and is not used that way.

Other similar fixed expressions with na:

na sastanku – at a meeting
na predavanju – at a lecture
na koncertu – at a concert
na poslu – at work

So na prvom satu = at the first lesson / in the first class.

What case is prvom satu, and why is it used here?

Prvom satu is in the locative singular:

prvi sat → nominative (first class)
prvom satu → locative, used after certain prepositions, including na in the sense “at (some event/place)”.

Pattern:

Preposition na (meaning at, in (a class/meeting, etc.))
- locative case
→ na prvom satu (at the first lesson).

Other examples:

na drugom katu – on the second floor
na fakultetu – at university
na koncertu – at the concert

Why is it između rečenica, and what case is rečenica here?

The preposition između (between) always takes the genitive case.

rečenica – nominative singular (a sentence)
rečenica – genitive plural has the same form in writing but different function here

In context između rečenica means between (the) sentences and rečenica is genitive plural:

između
- genitive plural → između rečenica

Other examples:

između kuća – between the houses
između knjiga – between the books
između prijatelja – between (the) friends

Why is razliku in that form, and how does razumjeti razliku između X i Y work?

Razliku is the accusative singular of razlika (difference).

The verb razumjeti (to understand) takes a direct object in the accusative:

razumjeti nešto – to understand something

So:

razumio (sam) razliku – I understood the difference
nisam bio razumio razliku – I had not understood the difference

Then you specify which difference with između X i Y:

razliku između rečenica radio sam i bio sam radio
= the difference between the sentences radio sam and bio sam radio

Could I just say Na prvom satu nisam razumio razliku… instead of nisam bio razumio?

Yes. That’s actually what most native speakers would say in everyday speech.

Na prvom satu nisam razumio razliku…
= perfectly natural, simple past meaning I didn’t understand the difference in the first lesson/class.

Using nisam bio razumio is:

More explicit about “earlier in the past” relative to some later moment
More formal / literary, especially in standard Croatian

If the speaker is female, does the form razumio change?

Yes. The past participle agrees in gender and number with the subject.

For a female speaker:

Nisam bila razumjela razliku između rečenica radio sam i bio sam radio.

Changes:

bio razumio (masculine) → bila razumjela (feminine)
- bio/bila – past participle of biti
- razumio/razumjela – past participle of razumjeti

Likewise:

Male: Radio sam cijeli dan. – I (male) worked all day.
Female: Radila sam cijeli dan. – I (female) worked all day.

Is the word order Na prvom satu nisam bio razumio razliku… fixed, or can it be changed?

The word order in Croatian is relatively flexible, but:

Clitics like nisam, sam, je must stay in second position of the clause.
You can move phrases around to change emphasis, as long as that rule is respected.

Some possible variants:

Razliku između rečenica radio sam i bio sam radio nisam bio razumio na prvom satu.
(emphasis on the difference itself)
Nisam bio razumio razliku između rečenica radio sam i bio sam radio na prvom satu.
(here na prvom satu gets pushed toward the end; still understandable)

But for clear, neutral style, Na prvom satu nisam bio razumio razliku… is a very natural choice.

How would I say the same thing in a more everyday, natural way?

A very natural, conversational version would be:

Na prvom satu nisam razumio razliku između rečenica radio sam i bio sam radio.

Or slightly more relaxed:

Na prvom satu nisam skužio razliku između rečenica radio sam i bio sam radio.
(skužio = colloquial “got / figured out / realized”)

The original with nisam bio razumio just sounds a bit more formal or bookish, because of the pluperfect.